As a result of decades of declining populations of Southern Flounder (Paralichthys lethostigma), some fishery-management agencies have begun producing flounder for stock-enhancement. One problem is that normal pigmentation may not develop in hatchery fish. The most prominent form of malpigmentation is pseudoalbinism, in which the eyed side of the fish is almost entirely white, which makes them exceptionally vulnerable to predators. At one hatchery, we determined that 30% of production were pseudoalbinos. In contrast, malpigmentation rates in our laboratory averaged 5%. We compared hatchery and laboratory rearing protocols to design experiments to identify ways to improve hatchery methods so that malpigmentation is minimized or eliminated. Among several differences, larval diet was identified as the top candidate for experimentation.
Experiments were conducted at the University of Texas Marine Science Institute’s Fisheries and Mariculture Laboratory to compare the two diets: non-enriched Artemia (hatchery) vs. Artemia enriched with Algamac DHA10 (laboratory). Five female flounder were strip-spawned, and 10,000 eggs from each spawn were divided equally between two rearing tanks (10 tanks in all), with one tank for each female assigned to each diet treatment. The larvae were reared to 55 days posthatching (dph), when the tank-wise malpigmentation rate was determined based on 100 individuals. In addition, malpigmentation rate was assessed for five tanks of flounder reared in the production hatchery through 55 dph for comparison with the laboratory results.
Malpigmentation rates from the laboratory experiment were 18.4% + 14.8% (mean + s.d.) from the non-enriched diet and 4.4% + 2.4% from the enriched diet. Hatchery-produced fish had a malpigmentation rate of 26.7% + 4.3%). Therefore, Artemia enrichment significantly improved the malpigmentation rate in the laboratory. For the subsequent production season (now underway), the hatchery changed the larval diet to enriched Artemia, and malpigmentation rate will be measured for 10 tanks to determine how well the laboratory results transfer to the hatchery. Some of the results may be available by the time of this presentation.
But, malpigmentation rates in the laboratory using non-enriched Artemia (18.8%) were lower than the rates observed in the hatchery (26.7%), and the 14% decrease due to the enriched diet would not bring the hatchery malpigmentation rate in line with the laboratory. This indicates that additional factors contribute to malpigmentation in the hatchery. Differences in light intensity and spectrum used in the hatchery vs. laboratory were examined. The hatchery used LED illumination at higher intensities (405 + 66 lux) than the fluorescent lighting used in the hatchery (274 + 22 lux) and the two light sources have very different spectral characteristics. Malpigmentation rates for the non-enriched diet (both locations included) showed a positive linear relationship with light intensity, suggesting that higher light intensities may increase malpigmentation rates. The effect of light intensity on malpigmentation rate is being tested in experiments that are currently in progress.